How long is too long when waiting in line before leaving?

Needs far more context.

I queued up buy lunch for about 5 seconds until walking away after I'd considered the length of the line and the fact that there's another lunch place I like that's often not as busy 100m away.

I've queued for many hours to catch a delayed plane flight.

Conceivably you could get me to queue for months or a year with a big enough carrot ("for some crazy reason we're giving away pretty nice houses to anyone who queues for them")

Depends what you're waiting for. Personally I'd say about 15-20 minutes is the max for something you can easily get elswhere (like food or many consumer items), 30-40 minutes would be a good maximum for methods of transport wherever there are alternative routes available, and you should only wait indefinitely if you're 100% certain whatever you're waiting for is only available at that location on that day/at that time.

there's a whole discipline devoted to this subject, aptly named "queuing theory."

personally, i always wait because I feel more regret if I later find out the line accelerated than I do self-satisfaction if I find out the line barely budged.

If you don't need the thing, why are you waiting in line to begin with? If you don't have anything better to do... wait.

If it’s open ended, 15m.

how valuable is your time?